1. Field of the Invention
This invention relates to dynamoelectric machines having a water-cooled rotor, and in particular, to a seal arrangement for a discharge chamber of such a rotor.
2. DESCRIPTION OF THE PRIOR ART
Large turbine generators are usually of the inner-cooled or direct cooled construction in which a coolant fluid is circulated through duct means in the stator and rotor slots in direct thermal contact with the current carrying conductors inside ground insulation. This type of construction provides an effective cooling system and has made it possible to greatly increase the maximum rating obtainable in large generators without exceeding the permissible limits of physical size. The coolant used in these machines has usually been hydrogen, which fills the gas-tight housing and is circulated by a blower on the rotor shaft through the ducts of the stator and rotor windings and through ducts in the stator core.
The maximum ratings required in large generators have continued to increase, making it necessary to further improve the cooling of these machines in the larger sizes. A substantial improvement in cooling can be obtained by the use of a more efficient cooling fluid, such as water. This has been done in stators by circulating water through the ducts of the stator winding. A further improvement can be obtained by applying a liquid coolant such as water to the rotor by circulation of the water through passages and the rotor windings.
Many problems are involved, however, in circulating a liquid coolant through the passages of the rotor of a large generator rotating at a high speed, usually 3600 revolutions per minute (in the U.S.). One of the most difficult problems is that of introducing the liquid into the rotor and discharging it therefrom. The liquid is preferably introduced along the axis of the shaft where the centrifugal force on the liquid is at a minimum, and is discharged through radial passages into the rotor shafts. A relatively large volume of liquid must be introduced into the rotor under sufficient pressure to maintain the desired flow rate to the rotor, and the same liquid is discharged from the rotor at high velocity and under high pressure into a stationary cooling discharge chamber from which it is drained. It is apparent that in a liquid cooled rotor suitable seals must be provided at the discharge passages. However, the provision of such seals for the discharge chamber poses a difficult problem because of the high velocity of the liquid, the pressures involved, and especially the large diameter of the rotor when the discharge chamber is located between the generator and the turbine.
Conventional seals for rotating shafts have serious disadvantages when applied to a discharge passage of a large dynamoelectric machine shaft rotating at high speeds. Labyrinth seals are well known in the art; however, because of the large clearances required between the seal housing and the rotating shaft, labyrinth seals are ineffective for liquids having a low viscosity, such as water, which results in an excessively large leakage flow through the seal, especially at large diameters.
Friction or face-type seals are also well known in the art. These seals, however, are impractical for sealing the discharge chambers of a liquid cooled rotor at the high rubbing velocities involved, which may be in excess of 20,000 feet per minute. Such rubbing velocities result in a very rapid wear with excessive heating due to friction loss, and are thus inappropriate for use to seal the discharge chambers of the liquid cooled rotors during high speed operation.
Fluid film gland seals using stationary seal rings are more suitable for the discharge chamber of the rotor during operation under the difficult conditions involved at high speeds. The known single flow type of circumferential ring seal, however, would allow excessive leakage of liquid through the clearance space between the seal rings and the shaft because of low viscosity of water. Since the coolant liquid must be decontaminated and must be treated to keep the oxygen content at a level that does not cause corrosion in the interior of the rotor windings, the leakage and subsequent loss of large amounts of coolant water is undesirable. The loss of this treated water would require that a large amount of treated make-up water be provided, requiring a larger water circulation system and an increased amount of expensive treatment equipment which would increase the manufacturing and operation costs. Leakage of the treated coolant must therefore be minimized.
The prior art provides a fluid film gland seal that utilizes a stationary seal ring encircling the shaft adjacent the coolant discharge chamber with a small clearance. In order to minimize leakage of the coolant liquid out of the discharge chamber through this clearance, a sealing liquid is introduced into the clearance between the ring and shaft. The sealing liquid is maintained at a pressure not exceeding the pressure of the coolant liquid in the discharge chamber. A small amount of the coolant liquid may therefore escape through the clearance around the shaft, but the amount of coolant liquid escaping is minimized and contamination of the coolant liquid by the sealing liquid is prevented. Several chambers having predetermined pressures therein are provided adjacent to the seal ring to prevent the liquid from escaping from the atmospheric chamber. In this way a very efficient seal is provided for a large volume of liquid with a high velocity and pressure. U.S. Pat. No. 3,733,501, issued to P. R. Heller, and assigned assignee of the present invention, discloses an example of the prior art using a gland seal of this type.
An even more efficient gland seal for sealing the discharge chamber of a water cooled rotor is that disclosed and claimed in U.S. Pat. No. 3,831,046, issued to L. P. Curtis et al., and assigned to the assignee of the present invention. This seal disposes the gaseous fluid within the discharge chamber in order to minimize fluid friction losses between the rotor surface and the discharge coolant liquid in the seal chamber. In order to prevent contamination of the coolant liquid, a stationary seal ring is provided in an annular chamber adjacent the discharge chamber. The seal ring encircles the shaft with a small predetermined clearance. In order to minimize leakage of the coolant liquid through the clearance space between the seal ring and the rotor shaft, a first sealing liquid is introduced through an opening in the seal ring into the clearance space between the ring and the shaft. This first sealing liquid is maintained at a predetermined pressure and is itself specially treated before use in the apparatus. A second sealing liquid is introduced through a separate opening in the sealing ring into the clearance between the sealing ring and the shaft. The second sealing fluid is maintained at a pressure not exceeding the pressure of the first sealing liquid. The first sealing liquid is disposed so as to be interposed between the partially filled coolant discharge and the second sealing liquid. Small amounts of the first sealing liquid may escape through the clearance space around the shaft into the coolant discharge chamber, but intermingling of the coolant liquid with the first sealing liquid would not be disadvantageous, since the first sealing liquid is treated in a manner similar to the coolant liquid.
However, since the first sealing liquid of the prior art gland seal described above is at a slightly higher pressure than the second sealing liquid, disposing the first sealing liquid between the coolant liquid and the second sealing liquid prevents the intermingling and the contamination of the coolant liquid by the second sealing liquid. Several chambers having predetermined pressures therein are provided to prevent the escape of liquid along the shaft. In this way a very effective seal is provided for handling large volumes of liquid coolant at high pressures and velocities. The seal increases the efficiency of the apparatus and prevents cavitation at the surface of the rotor.
Although the fluid film gland seals described above are efficient at the high operating velocities of the rotor, such seals are not as efficient at low rotating speeds or a standstill. Since for practical purposes the rotor must be kept filled with water at all times and also the discharge chamber is required to be isolated from the atmosphere, sealing liquid flow in these prior art seals must be maintained at all times. Consequently, at low speeds or at standstill where friction losses are small, large amounts of the specially treated sealing liquid may leak away from the sealing arrangement. This leakage is significant because it is the amount of the maximum sealing liquid flow, which is the flow at standstill, which determines the size and volume requirements of the sealing liquid feeding system. As stated earlier, contact seals are inefficient for use at high operational velocities due to excessive friction and rubbing wear which occur at these velocities. However, at low rotational velocities or at standstill, a contact seal is highly efficient.
Therefore, to minimize the maximum sealing fluid leakage flow through a large diameter fluid film gland seal and to minimize the size of the heating system of that seal, the sealing at standstill and at low speeds is improved by the incorporation of contact seals which are most efficient at standstill and at low rotational speeds. Such an arrangement is disclosed and claimed in the copending application of J. E. Albaric, Ser. No. 720,331, filed on 9/3/1976, and assigned to the assignee of the present invention. In that disclosure, a fluid film gland seal is provided to efficiently seal the discharge chamber during high rotational operation of the rotor, while a contact seal is provided to efficiently seal the discharge chamber at low rotating speeds and at standstill. Suitable control means for activating and deactivating the contact seal when the rotor speed reaches predetermined threshold values is also provided. In that disclosure, the contact seal is a segmented seal ring which is applied in response to the control means to insure sealing of the discharge chamber when the rotor speed reaches the predetermined threshold value.
Once the sealing at standstill and low speed is effectively taken care of, further reduction in the maximum rate of flow leakage through the seals with a reduction in the size of the system that circulates the lubricating water can only come from improvement of the sealing system effectiveness at operating speed.